docs: def get_join_cols(by_entry):
    """ helper function used for joins
    builds left and right join list for join function
    """
    left_cols = []
    right_cols = []
    for col in by_entry:
      if isinstance(col, str):
        left_cols.append(col)
        right_cols.append(col)
      else:
        left_cols.append(col[0])
        right_cols.append(col[1])
    return left_cols, right_cols